Obective lens design method, lens, and optical system, optical head, and optical disc apparatus using the same

ABSTRACT

When the DVD provided with the DVD substrate  2  having thickness t 2  of 0.6 mm is installed in the optical disc apparatus, the light beam  4  having wavelength λ 1 =655 nm is used as luminous flux of numerical aperture NA=0.63 to be condensed on the information surface  2   a  on the DVD substrate  2.  When the CD provided with the CD substrate  3  having thickness t 2  of 1.2 mm is installed in the optical disc apparatus, the light beam  5  having wavelength λ 2 =790 nm is effectively used as luminous flux of approximate numerical aperture NA=0.45 to be condensed on the information surface  3   a  on the DVD substrate  3.  The wavefront aberration caused by a thickness difference between the DVD substrate  2  and CD substrate  3  is canceled out by the chromatic aberration caused by a wavelength difference between the light beams  4  and  5.  Therefore, in spite of the difference in the transparent substrates, the light beams are suitably condensed respectively on the information surface  2   a  and  3   a.

This is a Continuation of application Ser. No. 10/670,209 filed Sep. 26,2003, which is a Continuation of application Ser. No. 10/330,289 filedDec. 30, 2002. The entire disclosure of the prior application is herebyincorporated by reference herein in its entirety.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention is related to Japanese Patent Application SerialNo. 2002-004993, and Japanese Patent Application Serial No. 2002-267451.They are hereby incorporated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a design method for an objective lensinstalled in a recording and reproducing apparatus of a multiplewavelengths optical system using plural kinds of monochromatic lightswhich is compatible with optical storage medium of different types suchas compact discs (CD) including a CD-R and digital versatile discs(DVD), an objective lens, a general-purpose multiple wavelengths lens, amultiple wavelengths optical system, an optical head, and an opticaldisc apparatus.

2. Related Background Art

Conventionally, a compatible optical disc apparatus capable ofreproducing different types of optical discs such as a CD and DVD hasbeen proposed. The CD and DVD (which will be hereinafter collectivelycalled an optical disc) both are provided with transparent substrates,and there is provided an information surface on one side thereof. Theoptical disc has two transparent substrates combined together, and theirinformation surfaces face each other, or the information surface facesthe other transparent substrate as a protection substrate. To reproduceinformation signals stored in the above optical disc, the optical discapparatus condenses a laser beam from a light source on the informationsurface of the optical disc through the transparent substrates. Asdetailed later, a wavelength of the laser beam differs between a CD andDVD. The optical disc apparatus uses an objective lens for condensingthe laser beam. Here, the thickness of the transparent substrateprovided with the information surface differs according to a type of theoptical disc (a difference in a laser beam wavelength). While thetransparent substrate of a CD is 1.2 mm in thickness, that of a DVD is0.6 mm. For the optical disc apparatus to reproduce optical discs ofdifferent types, it is required to condense the laser beam on theinformation surface while the thickness of the transparent substratediffers with the type of the optical disc. Besides, a new optical discapparatus using a blue laser of approximately 400 nm wavelength forreproducing information is recently proposed. Therefore, it is desirablefor the optical disc apparatus to be compatible with the new opticaldisc in addition to the CD and the existing DVD.

One approach for the above optical disc apparatus is to provide a pickupwith objective lens for different types of optical discs to change theobjective lens in accordance with the type of the optical disc in use,or to provide pickups for different types of the optical discs to changethe pickups in accordance with the type of the optical disc in use.However, for miniaturization and cost reduction, it is preferable tohave a single objective lens usable for any types of the optical disc.

Known as this kind of an objective lens is one disclosed in JapanesePatent Application Laid-Open No. H09-145995, for example. A lens surfaceof the objective lens disclosed therein is radially sectioned into morethan two loop zones, and every other loop zonal lens surfaces and theother every other loop zonal lens surfaces are different in refractingpower. For a laser beam of the same wavelength, the every other loopzonal lens surfaces condense the laser beam on the information surfaceof the optical disc (DVD) provided with the thin transparent substrates(0.6 mm), and the other every other zonal lens surfaces condense thelaser beam on the information surface of the optical disc (CD) providedwith the thick transparent substrates (1.2 mm), for example.

Another example is one disclosed in Japanese Patent ApplicationLaid-Open No. 2000-81566 (U.S. Pat. No. 6,118,594). It discloses theoptical disc apparatus using a laser beam of the shorter wavelength (635nm or 650 nm) for a DVD having the thinner transparent substrates whileusing a laser beam of the longer wavelength (780 nm) for a CD having thethicker transparent substrates. The optical disc apparatus is providedwith the objective lens used in common for those laser beams. Theobjective lens has a diffractive lens structure having a plurality ofminute loop zonal steps thickly formed on one side of a refractive lenshaving a positive refractive power. The diffractive lens structure isdesigned so that diffracted light of a laser beam having the shorterwavelength, and diffracted light of a laser beam having the longerwavelength, are condensed on the information surface respectively forthe DVD provided with the thinner transparent substrate, and for the CDprovided with the thicker transparent substrate. The lens is alsodesigned to condense diffracted light having identical diffractive orderfor both the diffracted lights. The reason why a laser beam having theshorter wavelength is used for the DVD is because a storage density ofthe DVD is higher than that of the CD, thus requiring a smaller beamspot. As well known, the diameter of an optical spot is proportional tothe wavelength and inversely proportional to the numerical aperture(NA).

There is also disclosed an objective lens of a loop zonal phasecorrection lens type provided with a loop zonal phase shifter formed onthe lens surface thereof in Japanese Patent Application Laid-Open No.2001-51192. First, a lens surface designed to have no wavefrontaberration for a laser beam having wavelength λ₁ of 640 nm is set as areference. A surface of the objective lens is then radially sectionedinto plural loop refractive zones, which are formed to havepredetermined steps (i-th step from a center of the lens is referred toas d_(i)) from the reference lens surface. Due to the step d_(i), eachof the refractive surfaces allows the laser beam of the DVD tophase-shift by integral multiple m_(i) of the wavelength λ₁ against thereference lens surface, thereby reducing wavefront aberration in a CDsystem.

In any of the above conventional techniques, it is possible to use acommon objective lens for both DVD and CD. It eliminates the need forchanging members including an objective lens for each use of a DVD orCD, which is effective in reducing costs and simplifying the structure.

However, the technique disclosed in Japanese Patent ApplicationLaid-Open No. H09-145995 uses different loop zonal lens surface of theobjective lens from a DVD to CD. Accordingly, large area remainsineffective for an incident laser beam, which extremely lowers light useefficiency.

Also, the technique disclosed in Japanese Patent Application Laid-OpenNo. 2000-81566 (U.S. Pat. No. 6,118,594) has a problem that diffractiveefficiencies for different wavelengths are unable to become 100% at thesame time because it uses the diffracted light by the diffractive lensstructure. In the diffractive lens, for a laser beam having the shorterwavelength (635 nm or 650 nm) used for a DVD and for a laser beam havingthe longer wavelength (780 nm) used for a CD, the diffractive efficiencyis designed to become 100% at intermediate wavelength between the two,in order to make the diffractive efficiency well balanced for the laserbeams in use. Besides, the technique requires minute steps to be formedon the lens surface for the diffraction lens structure, which isvulnerable to processing error. In a case where the diffractivestructure fails to be formed as designed, it causes a decrease in thediffractive efficiently. When the diffractive efficiency decreases orwhen it does not reach 100%, it means incapability of condensing all theincident light on the information surface on the transparent substrateof the optical disc, which results in the light loss.

Further, while having the high light use efficiency, the loop zonalphase correction lens type as disclosed in Japanese Patent ApplicationLaid-Open No. 2001-51192 also has a problem. It sets the lens surfacedesigned to have no wavefront aberration for the DVD laser beam as areference surface, and forms the refractive surfaces in d_(i) step,m_(i) integral multiple of the wavelength λ₁ of the DVD laser beam,below the reference lens surface, in order to reduce the wavefrontaberration for the CD laser beam. However, setting the reference for theDVD and then merely forming steps from there does not sufficientlydecrease the wavefront aberration for the CD laser beam.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above problemsand an object of the present invention is thus to provide a designmethod for an objective lens which can condense light beams on aninformation surface with the lowered wavefront aberration and at thehigh light use efficiency, a lens and an optical system, an opticalhead, and an optical disc apparatus using the lens.

For the above object, the present invention provides a design method foran objective lens for plural types of optical storage medium havingtransparent substrates of different thickness, the objective lensreceiving a light beam of a different wavelength for each of the pluraltypes of optical storage medium and having a positive power to condense,by refraction, the light beam onto an information surface on each of thetransparent substrates of the plural types of optical storage medium, inwhich a lens surface of the objective lens is designed in such a mannerthat chromatic aberration caused by a difference in wavelength of eachof the light beams substantially cancels out spherical aberration causedby a thickness difference between the transparent substrates.

The present invention also provides a lens for an objective lens forplural types of optical storage medium having transparent substrates ofdifferent thickness, the objective lens receiving a light beam of adifferent wavelength for each of the plural types of optical storagemedium and having a positive power to condense, by refraction, the lightbeam onto an information surface on each of the transparent substratesof the plural types of optical storage medium, in which the lenssubstantially cancels out spherical aberration caused by a difference inthickness between the transparent substrates with chromatic aberrationcaused by a difference in wavelength λ of each of the light beams, bywhich the objective lens condenses the light beam onto the informationsurface with a Root Mean Square (RMS) wavefront aberration of no morethan 0.035λ, preferably no more than 0.033λ, and more preferably no morethan 0.030λ, or with a RMS wavefront aberration satisfying a followingformula:{square root}{square root over ((ΣW _(i) ²)/i)}≦0.028  Formula 1:(λ_(i)(i=1, 2, . . . ) is a wavelength of the i-th light beam, ΣW_(i) ²is sum of square of each RMS wavefront aberration for all wavelengths,and W_(i)·λ_(i) is a RMS wavefront aberration of light beam havingwavelength λ_(i)), for each of the plural types of optical storagemedium.

The present invention also provides an objective lens condensing lightbeams on the information surfaces, with the left side of Formula 1having a value of no more than 0.026, preferably no more than 0.025, andmore preferably no more than 0.023, or with a RMS wavefront aberrationratio of

W_(max)/W_(min)<1.8, preferably W_(max)/W_(min)<1.6, and more preferablyW_(max)/W_(min)<1.4, where W_(max) is a maximum RMS wavefront aberrationamong RMS wavefront aberrations of each of the light beams and W_(min)is a minimum RMS wavefront aberration among RMS wavefront aberrations ofeach of the light beams. Or, there is provided a lens for an objectivelens for plural types of optical storage medium, the objective lensreceiving a light beam of a different wavelength for each of the pluraltypes of optical storage medium and having a positive power to condense,by refraction, the light beam onto an information surface on each of thetransparent substrates of the plural types of optical storage medium,and the lens condensing the light beam onto the information surface witha Root Mean Square (RMS) wavefront aberration of no more than 0.035λ foreach of the plural types of optical storage medium.

Besides, in order to produce a suitable RMS wavefront aberration in amultiple wavelengths optical system including a multiple wavelengthslens condensing, by refraction, plural kinds of monochromatic lights,the present invention provides a multiple wavelengths optical system asdescribed below, using a fact that one focal point of one monochromaticlight differs from other focal points of other monochromatic lights. Thepresent invention provides a lens having a lens surface sectioned intoplural aspherical zones having different refractive power in a commonuse area for all monochromatic lights, in which the plural sectionedaspherical zones respectively have one common single focal pointcorresponding to an inherent wavelength of each of the monochromaticlights, one focal point corresponding to the inherent wavelength of oneof the monochromatic lights is arranged in different position with otherfocal points corresponding to the inherent wavelengths of othermonochromatic lights, each of the aspherical zones, with regard to eachof the monochromatic lights, differs in optical path length from eachother by approximately integral multiple of wavelength λ_(i) of each ofthe monochromatic lights, and when a difference between a maximum valueand a minimum value of a wavefront aberration of each of themonochromatic lights in each of the aspherical zones is (ΔV_(d)(λ_(i)))where d is an integral number of 1, 2 . . . , meaning each of theaspherical zones, and i is an integral number of 1, 2 . . . , meaningeach of the monochromatic lights, each ratio of the differences of eachof the monochromatic lights in each of the aspherical zones is between0.4 and 2.5.

In the above multiple wavelengths optical system, it is preferable thatthe difference in the wavefront aberration of each of the monochromaticlights having wavelength λ_(i) in each of the aspherical zones is nomore than 0.14λ. In a case where the plural wavelengths are dualwavelengths, it is applicable, for example, to a dual wavelengthsoptical system with the longer wavelength around 790 nm for CD and theshorter wavelength of 655 nm, to a dual wavelengths optical system withthe longer wavelength of around 655 nm and the shorter wavelength ofaround 405 nm, to a dual wavelength optical system with the longerwavelength of around 790 nm and the shorter wavelength of around 405 nm,and further to a triple wavelengths optical system using those threewavelengths. Especially for the dual wavelengths optical systems, thewavefront aberrations preferably have symmetric figures.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams to show the first embodiment of anobjective lens according to the present invention.

FIG. 2 is a diagram to show an example of a lens surface configurationaccording to the first embodiment shown in FIG. 1.

FIG. 3 is a diagram to explain optical path length in a optical systemconsisting of an objective lens and a transparent substrate of anoptical disk.

FIGS. 4A and 4B are graphs to show an example of a measurement result ofwavefront aberration according to the first embodiment shown in FIG. 1.

FIGS. 5A and 5B are diagrams to show a calculation result of opticalspots for different types of optical discs in an optical disc apparatusemploying the first embodiment shown in FIG. 1.

FIGS. 6A and 6B are graphs to show an example of a measurement result ofwavefront aberration in an objective lens according to the presentinvention, in accordance with the second embodiment.

FIGS. 7A and 7B are diagrams to show a calculation result of opticalspots for different types of optical discs in an optical disc apparatusemploying the objective lens according to the present invention, inaccordance with the second embodiment.

FIG. 8 is a diagram to show an embodiment of an optical head accordingto the present invention.

FIG. 9 is a diagram to show an embodiment of an optical disc apparatusaccording to the present invention.

FIG. 10 is a pattern diagram to show wavefront aberration for eachwavelength for light height.

FIG. 11 is a diagram to show wavefront aberration for each wavelengthfor light height in the second embodiment.

FIG. 12 is a diagram to show wavefront aberration for each wavelengthfor light height when using a lens disclosed in Japanese PatentApplication Laid-Open No. 2001-51192.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an optical disc apparatus, for example, aberration in an objectivelens is appropriately corrected for the first optical disc provided witha transparent substrate having a thickness of ti so as to suitablycondense laser beams on an information surface formed on the transparentsubstrate. When the second optical disc provided with a transparentsubstrate having a different thickness of t₂ is installed in the opticaldisc apparatus, due to the difference of the substrate thickness t₂ fromt₁, the objective lens and the transparent substrate of t₂ thicknessgenerate spherical aberration. Therefore, it is unable to suitablycondense the laser beams on a information surface formed on thetransparent substrate of t₂ in thickness.

On the other hand, when laser beams having different wavelengths areused for a optical system consisting of the objective lens andtransparent substrate, chromatic aberration is generated. The chromaticaberration in this embodiment is a difference in spherical aberrationsgenerated for each laser beam when an objective lens receives a laserbeam having different wavelengths. For example, chromatic aberrationgenerated when an objective lens receives a laser beam of 655 nmwavelength and that of 790 nm wavelength is the difference betweenspherical aberration generated when the objective lens receives thelaser beam of 655 nm wavelength and spherical aberration generated whenthe objective lens receives the laser beam of 790 nm wavelength. Thepresent invention reduces the wavefront aberration caused by adifference in thickness between substrates, using the chromaticaberration. More specifically, the present invention uses a laser beamhaving a different wavelength for each of optical discs respectivelyprovided with substrates having different thickness, and cancels out thespherical aberration caused by a difference in thickness betweensubstrates with the chromatic aberration caused by a difference inwavelength of each of laser beams so that the aberrations together fallin the allowable range for the substrate of any thickness.

On condition that the spherical aberration in the substrate thickness t₁is S_(A)(t₁), and that in the thickness t₂ is S_(A)(t₂), and thespherical aberration in the laser beam wavelength λ₁ is S_(A)(λ₁), andthat in the wavelength λ₂ is S_(A)(λ₂), the chromatic aberration causedby a difference in wavelength is expressed by the difference in thespherical aberration (S_(A)(λ₂)−S_(A)(λ₁)). The present inventiondesigns a lens surface possibly satisfying the following formula.S _(A)(t ₂)−S _(A)(t ₁)=−(S _(A)(λ₂)−S _(A)(λ₁))  Formula 2:

In this configuration, for any optical discs provided with substrateshaving different thickness, when using a laser beam having a wavelengthcorresponding to a substrate thickness, all the light rays of the laserbeam which have passed an objective lens and substrate take optical pathlength to suitably condense on an information surface of the substrate.

As detailed later, a lens according to a preferred embodiment of thepresent invention has such a configuration that a lens surface issectioned into plural aspherical zones, and each of the aspherical zoneshas one common single focal point corresponding to the inherentwavelength of one of the monochromatic lights, and one focal pointcorresponding to the inherent wavelength of one of the monochromaticlights is arranged in different position with other focal pointscorresponding to the inherent wavelengths of other monochromatic lights.

A case where a laser beam is condensed on an information surface 2 a ofa substrate 2 with an objective lens 1 will be explained hereinafterwith reference to FIG. 3. A surface A of the objective lens 1 is a lightincident side, and a surface B is a light output side. The informationsurface 2 a is on the reverse of the side facing the objective lens 1.

In FIG. 3, a laser beam entering the objective lens 1 is parallel light(thus, the optical system shown in FIG. 3 is a so-called infiniteoptical system), and there is schematically shown a light path of alight beam starting from the optical axis OA of the objective lens 1,passing through a point P₁ located vertically h distant (light height)apart from the optical axis OA, and reaching a point P₅ where it crosseswith the optical axis OA (a condensing point). Here, where an incidentpoint to the objective lens 1 on the light path is P₂, and an outputpoint from the objective lens 1 is P₃, and an incident point to thetransparent substrate 2 is P₄, and

-   -   The point P₁˜The incident point P₂:    -   Clearance=S_(1h), Refractive index=n₁    -   The incident point P₂˜The output point P₃:    -   Clearance=S_(2h), Refractive index=n₂    -   The output point P₃˜The incident point P₄:    -   Clearance=S_(3h), Refractive index=n₃    -   The incident point P₄˜The condensing point P₅:    -   Clearance=S_(4h), Refractive index=n₄.        an optical path length L_(h) from the point P₁ to the condensing        point P₅ is expressed by the following formula:        L _(h) =n ₁ ×S _(1h) +n ₂ ×S _(2h) +n ₃ ×S _(3h) +n ₄ ×S        _(4h)  Formula 3:        The optical path length L_(h) on the optical axis OA is when h=0        in Formula 3.

Formula 3 is applicable to any light height h. When aberration iscorrected, the condensing point P₅ for each light height h is on theinformation surface 2 a within allowable ranges. The present inventionuses a laser beam having different wavelengths respectively for each ofsubstrates having different thickness; therefore, the sphericalaberration and the chromatic aberration cancel each other out so thatthe condensing point P₅ for each light height h is on the informationsurface 2 a within each of the allowable ranges.

A technique of sectioning an area of a lens surface where both thewavelengths are used in common, into plural aspherical zones when usinga monochromatic light λ₁ of 790 nm wavelength for CD and a monochromaticlight λ₂ of 655 nm wavelength for DVD, for example, is as follows.According to the technique, optical path length of one aspherical zonediffers from that of another by integral multiple of the wavelengthλ_(i) of each of the monochromatic lights. Further, the differencebetween a maximum value and a minimum value of wavefront aberration foreach of the monochromatic lights in each of the aspherical zones isΔV_(d)(λ₁) and ΔV_(d)(λ₂) where d is an integral number of 1, 2 . . . ,meaning each aspherical section. With those conditions, ratio betweenthe differences of each of the monochromatic lights is between 0.4 and2.5, preferably between 0.5 and 2.0, in any aspherical section, thusproducing the RMS wavefront aberration of a whole lens within theallowable range for both the wavelengths. When the optical path length,when the light height h=0, is L₀, and the optical path length of eachlight height is L_(h), the wavefront aberration V_(h) is expressed bythe following formula:V _(h)=(L _(h) −L ₀)/λ_(i)  Formula 4:

FIG. 10 schematically shows comparisons between the wavefrontaberrations by a lens for CD wavelength and those for DVD wavelength. InFIG. 10, the horizontal axis shows light height, and the vertical axisshows wavelength aberration, and the upper half shows wavefrontaberration in each aspherical section for a CD, and the lower half showswavefront aberration in each aspherical section for a DVD as calculatedby the above formula. For example, in the first area of the asphericalsection, the difference between the maximum value and the minimum valueof the wavefront aberration within the aspherical section is defined asΔV_(d)(λ₁) and ΔV_(d)(λ₂). As obvious from a later embodiment, accordingto the present invention, ratio of the differences falls in the range of0.4 through 2.5 in any aspherical section. Namely, in the presentinvention, each aspherical section has uniform distribution of thewavefront aberration for any wavelength, which is different fromconventional techniques configuring a lens surface based on onewavelength and correcting wavefront aberration in the other wavelengthusing phase lag. The integral multiple in the common use area ispreferably in the range between 0 and ±10, and more preferably inbetween 0 and ±5, provided that the multiple number in at least adjacentaspherical zones is not 0.

A multiple wavelengths lens according to the present invention iscapable of obtaining more suitable optical properties in each wavelengthby setting the difference between the maximum value and the minimumvalue of the wavefront aberration for each wavelength to be no more than0.14λ_(i) (for example, no more than 110.6 nm for 790 nm wavelength, and91.7 nm for 655 nm wavelength), preferably no more than 0.12λ_(i), andmore preferably no more than 0.10λ_(i), in each area of any asphericalzones.

Further, according to the present invention, when a multiple wavelengthslens on which the wavefront aberration for each wavelength becomesapproximate symmetric figures is employed in a dual wavelengths opticalsystem, two wavelengths are well balanced, thereby further reducing theRMS wavefront aberration.

Regarding the reduction of RMS wavefront aberration, the RMS wavefrontaberration for a CD is determined by the wavefront aberration only in acommon use area for a DVD and CD which is up to 1.58 mm light height inFIG. 10. On the other hand, there is a DVD sole use area (the range of1.58 to 2.02 mm light height in FIG. 10) outside of the common use area,and the RMS wavefront aberration for a DVD is determined by thewavefront aberration both in the common use area and in the sole usearea. Therefore, in a case of the DVD, even if the wavefront aberrationin the common use area is little too large, by correcting the wavefrontaberration for the DVD in complete disregard for those for the CD in theDVD sole use area, it is possible to sufficiently reduce the RMSwavefront aberration for the DVD to fall in the allowable range. Forinstance, in the diagram shown in FIG. 10, in the common use area forthe DVD and CD, the wavefront aberration for the DVD is 0 to −0.106λ,and that for the CD is 0 to +0.088, which is, the wavefront aberrationfor the CD is smaller than that for the DVD. The wavefront aberration inthe DVD sole use area is −0.052λ. The RMS wavefront aberration for theDVD is thus 0.0212λ RMS, and that for the CD is 0.0222λ RMS; therefore,the RMS wavefront aberrations for the DVD and for the CD areapproximately equal. In this manner, in order to produce the same RMSwavefront aberrations for the DVD and CD, it is effective to make thewavefront aberration for the CD smaller than that for the DVD in the DVDand CD common use area, and then adjust the RMS wavefront aberration forthe DVD by reducing the aberration in the DVD sole use area. Likewise,for producing the RMS wavefront aberrations of the different ratiobetween the DVD and CD, it should be counted that the wavefrontaberration for the DVD can be reduced in the sole use area even when theaberration generated in the common use area is little too large.

The above embodiment of the present invention makes it possible to formappropriate optical spots on the information surface for any types ofthe optical discs provided with the substrates having differentthickness. The present invention is applicable to a case where the discsubstrates do not have different thickness, which is, a case with thesame substrate thickness and different wavelengths, by making thecondensing points P₅ respectively fall within allowable ranges. Further,the present invention is not limited to be applied to optical storagemedium, but also applicable to a case where laser beams having differentwavelengths are passed through a single lens or optical system foroptical communication.

In the following, preferred embodiments of the present invention will beexplained with reference to the accompanying drawings, taking two typesof optical discs provided with transparent substrates having differentthickness, that are, a DVD and CD, as an example. A lens according tothe first embodiment of the present invention is made by forming resinmade of amorphous polyolefin by injection molding because it is simplein manufacturing. A lens according to the second embodiment has arefractive index of glass, and it can have a refractive index of plasticresin when lens material is plastic resin.

FIGS. 1A and 1B are diagrams to show the first embodiment of anobjective lens according to the present invention, and FIG. 1A is for aDVD, and FIG. 1B is for a CD. In FIGS. 1A and 1B, reference symbol 1denotes an objective lens according to the present embodiment, 2 denotesa transparent substrate of a DVD (which will be hereinafter referred toas a DVD substrate), 3 denotes a transparent substrate of a CD (a CDsubstrate), and 4 and 5 denote laser beams.

In FIG. 1A, the optical lens 1 is mounted in an optical head (not shown)of an optical disc apparatus. A DVD is installed in the optical discapparatus, and the objective lens 1 condenses the laser beam 4 as aparallel light, thereby processing recording or reproducing. Here, theDVD substrate 2 has the thickness t₁ of 0.6 mm, and the laser beam 4 haswavelength λ₁=655 nm, being a luminous flux having numerical aperture(NA)=0.63. Under such conditions, the laser beam is condensed on theinformation surface 2 a formed on the DVD substrate 2 on the reverse ofthe side facing the objective lens 1.

FIG. 1B shows a case installing a CD in the same optical disc apparatusas above, and processing recording and reproducing using the sameobjective lens 1. Here, the CD substrate 3 has the thickness t₂ of 1.2mm. Though the laser beam 5 has wavelength λ₂=790 nm, being a luminousflux having numerical aperture (NA)=approximately 0.63, luminous flux ofNA=0.47 is substantially condensed on the information surface 3 a on theCD substrate 3, and luminous flux of NA=approximately 0.47 to 0.63 shownby hatching which goes through a path apart from an optical axis OA ofthe objective lens is not condensed on the information surface 3 a. Thelens area where NA is approximately up to 0.47 is thus a common use areafor the DVD and CD.

As explained above, in order to reduce the aberrations for both DVD andCD so as to produce appropriate optical spots on the informationsurfaces 2 a and 3 a, the first embodiment configures a lens surface ofthe objective lens 1 so that optical path length L_(h) expressed byFormula 5 has a value within the allowable range for any light height hin both cases for DVD and CD. An example of such lens surfaceconfiguration will be explained hereinafter with reference to FIG. 2.

In FIG. 2, for a light output side B of the objective lens 1, a point atlight height h is c, and a point on the light output side B parallel tothe optical axis OA from the point c is d. The surface configuration ofthe light output side B is expressed with a distance Z_(B) between thepoints c and d for any light height h by the following formula:

Formula 5:$Z_{B} = {\frac{{Ch}^{2}}{1 + \sqrt{1 - {\left( {K + 1^{2}} \right){Ch}^{2}}}} + {A_{4} \cdot h^{4}} + {A_{6} \cdot h^{6}} + {A_{8} \cdot h^{8}} + {A_{10} \cdot h^{10}}}$where C=−0.12301 K=3.312138

-   -   A₄=0.01628151 A₆=−0.004311717    -   A₈=0.000682316 A₁₀=−0.00004157469

Substitution of numeral values into the above factor C, K, A₄, A₆, A₈,and A₁₀ in Formula 5 gives a value of the distance Z_(B) for any lightheight h (∫0) as negative, which means that the point d on the lightoutput side B is positioned closer to the input side (left side in FIG.2) compared with the point c, that is, a surface apex e of the lightoutput side B through which the optical axis OA passes. On the contrary,when the distance Z_(B) is a positive value, it means that the point dis positioned at the right side of the point e.

Now, for a light incident side A of the objective lens 1, a point atlight height h is a, and a point on the light incident side A parallelto the optical axis OA from the point a is b. The lens surface of thelight incident side A is configured so that the light height h (mm) anda distance Z_(A) (mm) between the points a and b for the light height hhave relations as shown in the following table: TABLE 1 h(mm) ZA(mm)0.00 0.000000 0.02 0.000095 0.04 0.000378 0.06 0.000851 0.08 0.0015130.10 0.002365 0.12 0.003405 0.14 0.004635 0.16 0.006055 0.18 0.0076640.20 0.009462 0.22 0.011451 0.24 0.013629 0.26 0.015998 0.28 0.0185570.30 0.021308 0.32 0.024249 0.34 0.027382 0.36 0.030707 0.38 0.0342240.40 0.037934 0.42 0.041838 0.44 0.045936 0.46 0.050229 0.48 0.0547180.50 0.059403 0.52 0.065640 0.54 0.070725 0.56 0.076008 0.58 0.0814900.60 0.087172 0.62 0.093056 0.64 0.099141 0.66 0.105429 0.68 0.1119210.70 0.118619 0.72 0.125522 0.74 0.132634 0.76 0.139954 0.78 0.1488550.80 0.156597 0.82 0.164550 0.84 0.172714 0.86 0.181092 0.88 0.1896840.90 0.198492 0.92 0.207517 0.94 0.216760 0.96 0.226225 0.98 0.2359111.00 0.245821 1.02 0.255956 1.04 0.266319 1.06 0.276910 1.08 0.2877331.10 0.298787 1.12 0.310076 1.14 0.321601 1.16 0.333364 1.18 0.3453671.20 0.357611 1.22 0.370098 1.24 0.382830 1.26 0.395809 1.28 0.4090371.30 0.422515 1.32 0.436245 1.34 0.450229 1.36 0.463024 1.38 0.4775311.40 0.492299 1.42 0.507330 1.44 0.522625 1.46 0.538186 1.48 0.5525591.50 0.568667 1.52 0.585050 1.54 0.600234 1.56 0.617173 1.58 0.6343951.60 0.653186 1.62 0.670934 1.64 0.688969 1.66 0.707293 1.68 0.7259081.70 0.744816 1.72 0.764020 1.74 0.783521 1.76 0.803322 1.78 0.8234241.80 0.843830 1.82 0.864540 1.84 0.885559 1.86 0.906886 1.88 0.9285251.90 0.950477 1.92 0.972744 1.94 0.995329 1.96 1.018233 1.98 1.0414592.00 1.065009 2.02 1.088887 2.04 1.113094 2.06 1.137635 2.08 1.1625122.10 1.187732 2.12 1.213298

The light output side B of the objective lens 1 as expressed by Formula5 and the light incident side A as shown in sequence data of Table 1both have continuous aspherical surfaces. Also, the distance between thesurface apexes f and e on the optical axis of the objective lens 1, thatis, center thickness t₀, is 2.2 mm. The refractive index n forwavelength λ₁=655 nm (DVD) is 1.54014, and the refractive index n forwavelength λ₂=790 nm (CD) is 1.5365.

-   -   (i) Here, an allowable value for the aberration for evaluation        is the RMS wavefront aberration of 0.035λ, preferably 0.033λ,        and more preferably 0.030λ for both DVD (wavelength λ₁=655 nm)        and CD (wavelength λ₂=790 nm) when an incident laser beam into        the objective lens 1 has the incident angle of 0°, which is,        when it is a parallel light to the optical axis OA. In the first        embodiment, the light output side B and light incident side A        are designed to have the above surface configuration so that the        wavefront aberrations for the DVD and CD are under such        allowable value.

While the first embodiment describes a case using two kinds of differentwavelengths λ₁ and λ₂, it is the same in a case using n kinds (n is anintegral number of 2 and above) of different wavelengths λ_(i)(i=1, 2, .. . , n).

-   -   (ii) In a case using n kinds of wavelength λ_(i), when each of        the RMS wavefront aberration, where an incident laser beam of        the wavelength λ_(i) has the incident angle of 0° is        W_(i)·λ_(i), the aberration satisfies the following formula:        {square root}{square root over ((ΣW _(i) ²)/i)}≦W ₀,  Formula 6:        where the wavelength of i-th light beam is λ_(i) (i=1, 2, . . .        ), sum of square of each RMS wavefront aberration for all the        wavelengths is ΣW_(i) ², and the RMS wavefront aberration of        light beam having wavelength λ_(i) is W_(i)·λ_(i). The allowable        value W₀ here is 0.028, preferably 0.026, and more preferably        0.025, further preferably 0.023. In the first embodiment where        the RMS wavefront aberration for a DVD is W₁, that for a CD is        W₂, and i=1, 2, the Formula 6 becomes as follows:        {square root}{square root over ((W ₁ ² +W ₂ ²)/2)}≦W ₀  Formula        7:    -   (iii) In a case using laser beams having n kinds of different        wavelengths λ_(i), when the maximum RMS wavefront aberration is        W_(max) and the minimum RMS wavefront aberration is W_(min), of        the n kinds of different wavelength λ_(i), it satisfies the        following formula:        1≦W _(max) /W _(min) <W _(th)        The allowable value W_(th) in this case is 1.8, preferably 1.6,        and more preferably 1.4. In the case of the first embodiment,        one of the RMS wavefront aberrations W₁ for a DVD and the RMS        wavefront aberration W₂ for a CD is the maximum RMS wavefront        aberration W_(max), and the other is the minimum RMS wavefront        aberration W_(min).

FIGS. 4A and 4B show calculation results regarding the RMS wavefrontaberration in the first embodiment. In the graph shown in FIGS. 4A and4B, the horizontal axis is image height (mm), and the vertical axis isRMS wavefront aberration.

FIG. 4A shows the RMS wavefront aberration for a DVD (wavelength λ₁=655nm), and when the image height is 0 mm, the RMS wavefront aberration is0.02130λ₁. FIG. 4B shows the RMS wavefront aberration for a CD(wavelength λ₂=790 nm), and when the image height is 0 mm, the RMSwavefront aberration is 0.02410 λ₂.

The above values are now substituted into each of the above formulas forthe evaluation. The results are as follows:

-   -   (i) First, for the DVD and CD, the RMS wavefront aberrations are        respectively 0.02130λ and 0.02410λ, which are smaller than the        allowable values of 0.035λ, 0.033λ, or 0.030λ    -   (ii) For the DVD and CD, from the Formula 7,

Formula 8:$\sqrt{\left( {W_{1}^{2} + W_{2}^{2}} \right)/2} = {\sqrt{\left( {0.02130^{2} + 0.02410^{2}} \right)/2} = 0.02274}$therefore, the value given is under the allowable value of 0.028 (orpreferably 0.026, 0.025, or 0.023).

-   -   (iii) For the DVD and CD, W_(max)/W_(min) is given as follows.    -   W_(max)/W_(min)=0.02410/0.02130=1.1315        Accordingly, it is under the allowable value of 1.8, 1.6. or        1.4.

FIGS. 5A and 5B show calculation results regarding optical spots on theinformation surfaces of the DVD and CD when using the objective lens 1provided with the light output side B having the surface configurationexpressed by Formula 5, and the light incident side A having the surfaceconfiguration shown in Table 1. In FIGS. 5A and 5B, the horizontal axisshows a position in a vertical direction expressed by the distance (mm)from the reference point where the optical axis is on the informationsurface, and the vertical axis shows a relative light intensity in eachposition when a light intensity at the reference point (=0 mm) is 1.

FIG. 5A shows optical spots for a DVD, and an optical spot with therelative light intensity of 1/e²(=13.5%)has a diameter φD of 0.85 μm.FIG. 5B shows optical spots for a CD, and an optical spot with therelative light intensity of 1/e₂ has a diameter φc of 1.37 μm. Thus,both for the DVD and CD, optical spots are appropriately produced on theinformation surface.

Now, the second embodiment of the objective lens according to thepresent invention will explained hereinbelow.

In the second embodiment whose basic configuration is the same as thefirst embodiment, the light incident side A is radially sectioned intoplural zones from the optical axis, and each zone surface is configuredso that the aberrations for both DVD and CD are reduced to fall withinthe allowable value.

The surface configuration of the light incident side A according to thesecond embodiment will be explained hereinafter with reference to FIG.2. The distance between the points a and b in a j-th zone from theoptical axis OA in the direction of the light height h (in the radialdirection) on the light incident side A is expressed by the followingfunction Z_(Aj)

Formula 9:$Z_{Aj} = {B + \frac{{Ch}^{2}}{1 + \sqrt{1 - {\left( {K + 1^{2}} \right){Ch}^{2}}}} + {A_{4} \cdot h^{4}} + {A_{6} \cdot h^{6}} + {A_{8} \cdot h^{8}} + {A_{10} \cdot h^{10}} + {A_{12} \cdot h^{12}} + {A_{14} \cdot h^{14}} + {A_{16} \cdot h^{16}}}$The light height h in Formula 9 is that in the j-th zone.

The following table shows the range of h and the constant B, C, K, A₄,A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆ for each zone in Formula 9 to reduce theaberrations for both DVD and CD to be within the allowable value range.TABLE 2

B C K A4 1     0˜0.464667   0 4.45390E−01 −6.67483   0.030017 20.464667˜0.687967   0.00108409 4.46999E−01 −8.52884E−01 −1.11390E−03 30.687967˜0.904685   0.00216818 4.45826E−01 −5.85171E−01   2.36910E−03 40.904685˜1.414529   0.00325226 4.46759E−01 −6.51167E−01   9.59140E−04 51.414529˜1.519145   0.00216818 4.28660E−01 −3.27869E−01   6.74850E−03 61.519145˜1.589366   0.00108409 4.42061E−01 −5.75461E−01   2.50970E−03 71.589366˜1.847991   0.00103073 4.45481E−01 −6.24870E−01   1.77360E−03 81.847991˜2.2 −0.00113744 4.45319E−01 −6.13552E−01   1.64620E−03 A6 A8A10 A12 A14 A16   1.026695 −14.364412 106.233381 −431.806672 895.812958−725.25403   8.21580E−03   9.75070E−03 −3.03780E−02 −4.03770E−021.80080E−01 −1.49931E−01 −5.00360E−03   4.59410E−03   3.04280E−03−8.16540E−03 6.55040E−03 −2.40830E−03   4.83750E−04   2.53240E−04−1.24670E−04 −1.01670E−04 7.59740E−05 −1.37590E−05   3.92010E−04−1.18360E−03 −4.24370E−04   7.93880E−05 1.45720E−04 −3.75090E−05  1.75070E−04 −2.24990E−04 −1.73810E−05   6.71230E−07 1.64740E−05−3.96660E−06   1.24130E−04 −7.34610E−05   2.76610E−05 −9.44330E−061.32750E−06 −6.17570E−08   1.15930E−04 −7.50480E−05   2.80220E−05−9.53270E−06 1.33720E−06 −6.23130E−08

The surface configuration Z_(B) of the light output side B according tothe second embodiment is expressed by the following formula:

Formula 10:$Z_{B} = {\frac{{Ch}^{2}}{1 + \sqrt{1 - {\left( {K + 1} \right){C^{2} \cdot h^{2}}}}} + {A_{4} \cdot h^{4}} + {A_{6} \cdot h^{6}} + {A_{8} \cdot h^{8}} + {A_{10} \cdot h^{10}}}$where

-   -   C=−0.0747792 K=15.7398 A₄=0.012308    -   A₆=−0.0037652 A₈=0.00068571 A₁₀=−0.000048284.

The distance between the surface apexes f and e on the optical axis ofthe objective lens 1, that is, the center thickness t₀, is 2.2 mm. Therefractive index n for wavelength λ₁=655 nm (DVD) is 1.604194, and therefractive index n for wavelength λ₂=790 nm (CD) is 1.599906.

Here, the allowable value for the aberration for evaluation is the sameas the first embodiment.

FIGS. 6A and 6B show calculation results regarding the RMS wavefrontaberration in the second embodiment. In the graph, the horizontal axisand vertical axis are the same as FIGS. 4A and 4B.

FIG. 6A shows the RMS wavefront aberration for a DVD (wavelength λ₁=655nm), and when the image height is 0 mm, the RMS wavefront aberration is0.01945λ₁. FIG. 6B shows the RMS wavefront aberration for a CD(wavelength λ₂=790 nm), and when the image height is 0 mm, the RMSwavefront aberration is 0.02525 λ₂.

FIG. 11 shows a calculation result regarding the wavefront aberration onthe lens in the common use area. The following table shows thedifference in the wavefront aberration and the ratio of the differences,in each of the aspherical zones: TABLE 3 Area 1 Area 2 Area 3 Area 4Area 5 Area 6 ΔV_(d)(λ655)(λ) 0.0955 0.0960 0.0963 0.0692 0.0939 0.0937ΔV_(d)(λ790)(λ) 0.0977 0.0973 0.0967 0.0701 0.0974 0.0976 ΔV_(d)(λ790)/1.02 1.01 1.00 1.01 1.04 1.04 ΔV_(d)(λ655) ΔV_(d)(λ655)/ 0.98 0.99 1.000.99 0.96 0.96 ΔV_(d)(λ790)

As shown in Table 3, in the common use area of 790 nm and 655 nm, theratio of the difference ΔV_(d)(λ790)(λ)/ΔV_(d)(λ655) of each wavefrontaberration falls in the range of 1.00 and 1.04. The ratioΔV_(d)(λ655/ΔV_(d)(λ790) is in the range of 0.96 and 1.00. Thedifference of wavefront aberration itself in each area is no more than0.14λ in both wavelengths. In the lens, the wavefront aberration isproduced at a positive side for 790 nm wavelength, and at a negativeside for 655 nm wavelength; also, the wavefront aberrations becomeapproximately symmetric.

There is generated the differences in the optical path length betweenthe adjacent aspherical zones sectioned centered on the optical axis.The differences are set to be integral multiple corresponding to eachwavelength. In the present embodiment, there are even number ofsectioned aspherical zones.

The above values are now substituted into each of the above formulas forthe evaluation, as is the case with the first embodiment. The resultsare as follows:

-   -   (i) First, for the DVD and CD, the RMS wavefront aberrations are        respectively 0.01945λ and 0.02525λ, which are smaller than the        allowable values of 0.035λ, 0.033λ, or 0.030λ    -   (ii) For DVD and CD, from the Formula 7,

Formula 11:$\sqrt{\left( {W_{1}^{2} + W_{2}^{2}} \right)/2} = {\sqrt{\left( {0.01945^{2} + 0.02525^{2}} \right)/2} = 0.02254}$therefore, the value given is under the allowable value of 0.028, 0.026,0.025, or 0.023.

-   -   (iii) For DVD and CD, W_(max)/W_(min) is given as follows.    -   W_(max)/W_(min)=0.02525/0.01945=1.298        Accordingly, it is under the allowable value of 1.8, 1.6. or        1.4.

FIGS. 7A and 7B show calculation results regarding optical spots on theinformation surfaces of a DVD and CD when using the objective lens 1provided with the light output side B having the surface configurationexpressed by Formula 10, and the light incident side A having thesurface configuration expressed by Formula 9 and shown in Table 2. Thehorizontal axis and vertical axis of the graph is the same as FIG. 5.

FIG. 7A shows optical spots for a DVD, and an optical spot with therelative light intensity of 1/e²(=13.5%) has a diameter φD of 0.89 μm.FIG. 7B shows optical spots for a CD, and an optical spot with therelative light intensity of 1/e² has a diameter φc of 1.30 μm. Thus,both for DVD and CD, optical spots are appropriately produced on theinformation surface.

In the second embodiment, the ratio is 0.96 to 1.04 in Table 3, and theRMS wavefront aberration for the DVD is 0.01945λ₁ and that for the CD is0.02525λ₂. However, it is possible to produce equal RMS wavefrontaberrations of around 0.022 to 0.023λ for the DVD and CD by increasingthe aberration for the DVD while reducing that for the CD in the commonuse area, as explained above with reference to FIG. 10.

The RMS wavefront aberrations for a DVD and CD disclosed in JapanesePatent Application Laid-Open No. 2001-51192, for example, are asfollows.

EXAMPLE 1

DVD: 0.001λ₁ CD: 0.047λ₂

EXAMPLE 2

DVD: 0.019λ₁ CD: 0.037λ₂

-   -   (λ₁=640 nm, and λ₂=780 nm)        Each of the above values is over the allowable value of 0.035λ.

Besides, when calculating the wavefront aberration of lens for eachwavelength in example 2 by using the lens data disclosed in the aboveapplication, as shown in the following table 4 and FIG. 12, the ratio ofthe difference, ΔV_(d)(λ655)/ΔV_(d)(λ790) is 0.21 through 33.44 andΔV_(d)(λ790)(λ)/ΔV_(d)(λ655)is 0.03 through 4.72, which are outside ofthe limits set for the present invention, thus being out of balance. Inaddition, while the wavefront aberration produced for the DVD is no morethan 0.14λ, that for the CD are large, and therefore the RMS wavefrontaberration of lens as a whole become large. TABLE 4 Area 1 Area 2 Area 3Area 4 Area 5 ΔV_(d)(λ655)(λ) 0.0415 0.0518 0.0242 0.1012 0.1906ΔV_(d)(λ790)(λ) 0.1489 0.1635 0.1142 0.1039 0.0057 ΔV_(d)(λ655)/ 0.280.32 0.21 0.97 33.44 ΔV_(d)(λ790) ΔV_(d)(λ790)/ 3.59 3.16 4.72 1.03 0.03ΔV_(d)(λ655)

When the values are substituted into the following formula:{square root}{square root over ((W ₁ ² +W ₂ ²)/2)}  Formula 12:it gives 0.0332 and 0.0294 respectively, both of which are over theallowable value of 0.028, preferably 0.026, more preferably 0.025, andfurther preferably 0.023. Further, the value of W_(max)/W_(min) forthose are 47 and 1.847 respectively, both of which are over theallowable value of 1.8, preferably 1.6, and more preferably 1.4.

As explained in the foregoing, the first and second embodiments arecapable of keeping the aberration within the allowable value because thelens surface has such a configuration that spherical aberrationgenerated by a difference in substrate thickness and chromaticaberration cancel each other out to bring the total aberration withinthe allowable value. On the other hand, the technique disclosed inJapanese Patent Application Laid-Open No. 2001-51192 reduces theaberration for a CD by phase-shifting the incident laser beam forintegral multiple of the wavelength of the DVD laser beam. Though thetechnique is able to sufficiently reduce the aberration for either oneof the wavelengths, it is unable to keep the aberration for all of thewavelengths at the same time within the allowable limit of such a smallvalue.

It is obvious from the optical spots shown in FIGS. 5A, 5B, 7A, and 7B,and the wavefront aberration shown in the graphs in FIGS. 4A, 4B, 6A,and 6B that, in the above embodiment, the spherical aberration caused bya difference in substrate thickness between DVD and CD, which is, 0.6 mmand 1.2 mm respectively, are canceled by the chromatic aberration causedby a difference in wavelength, 655 nm and 790 nm respectively, to reducethe total aberration. Besides, in the embodiments, the objective lens 1has the light incident side A whose surface configuration is given bythe sequence data in Table 1, Formula 9, and Table 2, and the lightoutput side B whose surface configuration is given by the asphericalsurface formulas expressed by Formula 5 and 10, which is different fromthe diffractive lens surface as disclosed in the conventional technique.Further, the objective lens 1 is able to condense approximately allluminous flux for numerical aperture (NA) necessary for recordation orreproduction, thereby obtaining the high light use efficiency.

In the above embodiment, the outer area of the objective lens 1 havingnumerical aperture of approximately NA=0.47 to NA=0.63 is used only fora DVD, and not used for a CD, as shown in FIG. 1. Therefore, it ispossible to form a thin film through which light of the 655 nm DVDwavelength passes and light of the 790 nm CD wavelength does not pass oneither one or both of the light incident side A and the light outputside B in the outer area. It is also possible to form a diffractiongrating not acting on light of the 655 nm wavelength, but acting onlight of the 790 nm wavelength on either one or both of the lightincident side A and the light output side B in the outer area. In thisconfiguration, the light use efficiency for light having the 790 nmwavelength can be reduced without reducing the light use efficiency forlight having the 655 nm wavelength.

When the present invention is applied to systems having differentnumerical apertures but not having the diaphragm according to thenumerical apertures, the lens receives excess luminous flux in a systemhaving smaller numerical aperture. Therefore, it is preferable to makearrangements such that the light passing through the outer area of thelens designed according to an optical system having larger numericalaperture does not have adverse effects on an optical system havingsmaller numerical aperture. For example, an amount of transverseaberration is preferably 0.015 mm and over.

Though the above embodiment has been given of the case using opticaldiscs of two types, a DVD and CD, the present embodiment is not limitedthereto, whereas it is applicable to the case using other optical discsof different types or optical discs of more than two types provided withsubstrates having different thickness. In these cases, laser beams areset to have different wavelengths, and a lens surface is configured sothat chromatic aberration cancels wavefront aberration out according tothe wavelength.

Further, the present invention is also effective in reducing aberrationwhen applied to the case where substrate thickness is the same butwavelength is different, causing large aberration with conventionallens.

FIG. 8 shows a preferred embodiment of an optical head using theobjective lens according to the present invention. In FIG. 8, referencesymbol 11 denotes a DVD laser, 12 a CD laser, 13 and 14 half prism, 15 acollimator lens, 16 a detection lens, 17 a light detector, 18 adiffraction grating, and 19 an actuator. The same elements as in FIG. 1are denoted by the same reference symbols.

In FIG. 8, in a case of recording or reproducing the DVD disc 2, the DVDlaser 11 is driven. A laser beam of the 655 nm wavelength generated inthe DVD laser 11 is reflected by the half-prism 13, passes through thehalf-prism 14, and enters the collimator lens 15. Becoming parallellight at the collimator lens 15, the laser beam enters the objectivelens 1 where it is condensed to form an optical spot on an informationsurface of the DVD disc 2. Then, the reflected light reflected by theDVD disc 2 becomes parallel light at the objective lens 1 to enter thecollimator lens 15. At the collimator lens 15, the parallel light thenbecomes condensed light, which passes through the half-prisms 14 and 13and the detection lens 16 to reach the light detector 17. Detectionoutput signals in the light detector 17 are delivered to a signalprocessing circuit (not shown) where information recording andreproducing signals, focus error signals, and tracking error signals areobtained. A system control circuit (not shown) then controls an actuatordrive circuit (not shown) to drive the actuator 19 so that the objectivelens 1 is positioned in an appropriate focus position and trackingposition based on the obtained focus error signals and tracking errorsignals.

On the other hand, in a case of recording or reproducing the CD disc 3,the CD laser 12 is driven. A laser beam of the 790 nm wavelengthgenerated in the CD laser 11 passes through the diffraction grating 18,is reflected by the half-prism 14, and enters the collimator lens 15.Becoming parallel light at the collimator lens 15, the laser beam entersthe objective lens 1 where it is condensed to form an optical spot on aninformation surface of the CD disc 3. Then, the reflected lightreflected by the CD disc 3 becomes parallel light at the objective lens1 to enter the collimator lens 15. At the collimator lens 15, theparallel light then becomes condensed light, which passes through thehalf-prisms 14 and 13 and the detection lens 16 to reach the lightdetector 17. Detection output signals in the light detector 17 aredelivered to a signal processing circuit (not shown) where informationrecording and reproducing signals, focus error signals, and trackingerror signals are obtained.

The tracking error signals in the case of CD disc 3 are obtained by±first order light, which is one of three beams, zero-order light and±first order light, diverged from the laser beam from the CD laser 12with the diffraction grating 18.

Based on the tracking error signals obtained as above and the focuserror signals, as is the case with the DVD disc 2, the actuator 19 isdriven to arrange objective lens 1 in an appropriate focus position andtracking position.

It is also possible to make an optical configuration in the opticalsystems common to both discs, such as the collimator lens 15 or thehalf-prism 14, instead of in the objective lens, so that they have thesame functions as the objective lens according to the present invention.Also, while not shown, another optical element having the same functionsas the objective lens of the present invention may be arranged on theoptical path from the half-prism 14 through the disc 2 or 3.

The collimator lens 15 is not necessary, and the present invention isalso applicable to an optical system of a so-called finite system.

FIG. 9 shows a preferred embodiment of an optical disc apparatus usingthe objective lens according to the present invention. In FIG. 9,reference symbol 20 denotes an actuator drive circuit, 21 a signalprocessing circuit, 22 a laser drive circuit, 23 a system controlcircuit, and 24 disc discriminating means. The same elements as in FIG.8 are denoted by the same reference symbols.

In FIG. 9, the configuration of the optical pickup apparatus is the sameas shown in FIG. 8.

First, the disc distinguishing means 24 distinguishes a type of a discloaded. Among methods for distinguishing the disc are a method detectingthe thickness of the disc substrate optically or mechanically and amethod detecting a reference mark preciously stored in the disc or adisc cartridge. Or, there is also a method reproducing disc signals withtentative disc thickness and type, and judging that it is a disc ofanother thickness and type if normal signals are not obtained. The discdistinguishing means 24 then transmits the result to the system controlcircuit 23.

When the result shows that the disc is the DVD disc 2, the systemcontrol circuit 23 transmits a signal for lighting the DVD laser 11 tothe laser drive circuit 22, and the DVD laser 11 lights by the laserdrive circuit 22. Thus, in an optical head, the laser beam having the655 nm wavelength reaches the light detector 17, as is the embodimentshown in FIG. 8. The light detector 17 then transmits detection signalsto the signal processing circuit 21 where information recording andreproducing signals, focus error signals, and tracking error signals aregenerated and transmitted to the system control circuit 23. The systemcontrol circuit 23 controls the actuator drive circuit 20 based on thefocus error signals and tracking error signals to drive the actuator 19,thereby moving the objective lens 1 in the focus direction and trackingdirection, which is called a servo circuit operation. By this operation,the focus control and tracking control are regularly processed, and theabove circuits and the actuator 19 operate to arrange the object lens 1in a right position to the DVD disc 2, thus suitably obtaining theinformation recording and reproducing signals.

On the other hand, when the result shows that the disc loaded is the CDdisc 3, the system control circuit 23 transmits a signal for lightingthe CD laser 12 to the laser drive circuit 22. The CD laser 12 thusgenerates the laser beam having the 790 nm wavelength. The subsequentoperations are the same as the case of the optical head shown in FIG. 8.The laser beam reaches the light detector 17, and the circuits and theactuator 19 process the servo operation to obtain the informationrecording and reproducing signals suitably, as is the case with the DVDdisc 2.

As explained in the foregoing, the present invention makes it possibleto condense by refraction all luminous flux of numerical aperture (NA)necessary for recordation and reproduction on a desirable position withaberration of as small as possible for more than one type of opticaldiscs provided with transparent substrate having different thickness,without using diffractive lens structure, thus increasing the light useefficiency. Besides, as also explained above, according to the presentinvention, in a multiple wavelengths optical system using pluralmonochromatic lights, each of sectioned aspherical surface has a singlefocal point corresponding to inherent wavelength of each of themonochromatic lights, and the focal points are respectively arranged indifferent positions. The present invention is applicable to an opticalsystem in an optical communication.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. An objective lens for plural types of optical storage medium havingtransparent substrates of different thickness, the objective lensreceiving a light beam of a different wavelength λ_(i) for each of theplural types of optical storage medium and having a positive power tocondense the light beam onto an information surface on each of thetransparent substrates of the plural types of optical storage medium;wherein the objective lens substantially cancels out sphericalaberration caused by a thickness difference between the transparentsubstrates with chromatic aberration caused by a difference in thewavelength of each of the light beams, by which the objective lenscondenses the light beam onto the information surface with a Root MeanSquare (RMS) wavefront aberration ratio of1≦W _(max) /W _(min)<1.8 where W_(max)is a maximum RMS wavefrontaberration among RMS wavefront aberrations of each of the light beamsand W_(min) is a minimum RMS wavefront aberration among RMS wavefrontaberrations of each of the light beams, for each of the plural types ofoptical storage medium, and the wavelength λ_(i) comprises at least twokinds of wavelength selected from a group of about 790 nm, 650 nm, and405 nm.
 2. An objective lens according to claim 1, wherein the value ofW_(max)/W_(min) is less than 1.6.
 3. An objective lens according toclaim 1, wherein the value of W_(max)/W_(min) is less than 1.4.
 4. Anobjective lens according to claim 1, wherein the lens condenses eachwavelengths of light onto an information surface with a RMS wavefrontaberration of no more than 0.035λ.
 5. An optical head using a lensaccording to claim
 1. 6. An optical head using a lens according to claim2.
 7. An optical head using a lens according to claim
 3. 8. An opticaldisc apparatus using a lens according to claim
 1. 9. An optical discapparatus using a lens according to claim
 2. 10. An optical discapparatus using a lens according to claim 3.